Visible authentication patterns for printed document

ABSTRACT

Techniques for determining authenticity of analog forms such as packaging or documents ( 117 ). One of the techniques determines whether the analog form has been made directly from a digital representation ( 903 ) or by photocopying or scanning an analog form. The technique makes the determination by comparing ( 911 ) an original digital representation of a portion of the analog form with a digital recording ( 203 ) of the portion from the analog form and measuring differences in features that are affected by the operations of photocopying or scanning. The original digital representation ( 105 ) and the analog form may have a “noisy”, i.e., random or pseudo random pattern. Such noisy patterns may further be used for other authentication purposes, such as determining whether the portion of the analog form that has the noisy pattern has been altered and to carry hidden messages. The noisy pattern may carry a logo or may be part or all of a barcode.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority from U.S. ProvisionalApplication 60/380,189, Method and Apparatus for Copy Protection withCopy-Detectable Patterns, having the same inventors as the presentapplication and filed May 14, 2002 and further claims priority from andis a continuation-in-part of U.S. Ser. No. 10/287,206, J. Zhao, et al.,Apparatus and methods for improving detection of watermarks in contentthat has undergone a lossy transformation, filed Nov. 4, 2002 now U.S.Pat. No. 6,782,116. The sections Watermarks that are embedded usingmessage-based keys and Using watermarks to locate alterations in digitaldocuments and analog documents made from digital documents from thatapplication are included in their entirety in the present application.All of U.S. Ser. No. 20/287,206 is further incorporated by referenceherein for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to security features in printeddocuments and more specifically to visible authentication patterns inprinted documents. The visible authentication patterns can be used todistinguish original printed documents from photocopies of those printeddocuments, to detect alterations in documents, and to carry hiddenand/or visible messages.

2. Description of Related Art

A prerequisite for a commercial society is being able to distinguishauthentic items from false or counterfeit items. With documents, thequestions that may need to be answered to determine a document'sauthenticity include the following:

-   -   Is the document an original or a copy of an original?    -   Has the document been altered since it was first made?    -   Is the document authorized?

Many techniques have been developed to make it reasonably possible toanswer these questions from the document itself. Techniques which makeit easier to determine whether a document is an original or a copyinclude the complex etchings used in paper money, the photographs usedin ID cards, and special papers. Special papers and inks have been usedto detect alterations. Techniques for showing that a document isauthorized have included signatures and seals.

With the advent of digital scanning and printing techniques, digitalwatermarks have been used to authenticate printed documents. A digitalwatermark is a message that is embedded in a digital representation of adocument by adding noise that carries the message to a graphic elementin the document. When used for authentication purposes, the message isgenerally invisible and can be read only if the location of theinformation making up the message in the graphic element is known. For asurvey of digital watermarking techniques, see Appendix A, beginning atcol. 17, line 27 of U.S. Pat. No. 6,345,104, Rhoads, Digital watermarksand methods for security documents, issued Feb. 5, 2002, which isincorporated herein by reference for all purposes. For an example of howdigital watermarks may be used for authentication purposes, see U.S.Pat. No. 6,243,480, Jian Zhao, Digital authentication with analogdocuments, issued Jun. 5, 2001.

Progress in the technology of copying has diminished the value of all ofthe techniques that permit one to determine from the appearance of adocument whether it is authentic. Because of this progress,counterfeiting not only of money and financial instruments, but also ofother documents such as ID cards and diplomas as well as of packagingand labels causes huge losses, ranging to 5% to 8% of worldwide sales ofbranded products, and endangers the reputation and value of the brandsthemselves. Moreover, the growth of the Internet drives the business ofcounterfeited documents (fake IDs, university diplomas, checks, and soon), which can be bought easily and anonymously from hundreds ofcompanies on the Web. As the precision of scanners, digital imagingsoftware and printers increase, the problem will only get worse.

What is needed as the scanners, digital imaging software, and printersget better is new ways of adding information to a document to make itpossible to determine whether it is an original or a copy, whether ithas been altered, and/or whether it has been authorized. Efforts in thisarea have included the following:

Embedding Multiple Watermarks in a Document

U.S. Pat. No. 6,332,031, Rhoads, et al., Multiple watermarkingtechniques for documents and other data, issued Dec. 18, 2001, disclosesembedding several watermarks in the document, each of them withdifferent properties or in different domains. If the document isphotocopied or scanned and printed in order to produce a counterfeit,the embedded watermarks will be altered or damaged. The properties ofthe watermark or the domain in which it is embedded will affect thedegree to which it is altered by the copying process. Thus, the relativedegree of alteration of each watermark can indicate whether the documentis an original or copy. The use of watermarks in this fashion has anumber of advantages:

-   -   It is flexible: a digital watermark can in theory be inserted        into any document, because it merely introduces unnoticeable        modifications to the document.    -   Because it is invisible, it can be used to determine the source        of the counterfeits.    -   The possible presence of watermarks forces the counterfeiter to        reproduce the whole document with very high fidelity.

The advantages of watermarks are also their disadvantages. Becausedigital watermarks used for security purposes are made by addinginvisible noise to a document, they often cannot be read where thedocument that contains them has been subject to wear and tear. Becausethey are hidden in the noise in a document, it is difficult to hide thewatermarks in documents such as banknotes where every element of thedesign is fixed and there is thus no room for noise.

Embedding Information that Cannot be Reproduced by a Photocopier

A document may contain a part which is invisible in the visible lightrange because it is printed with an ink that is visible in ultra-violetlight. The photocopier, which operates using visible light, cannotreproduce it. See U.S. Pat. No. 5,868,432, Mantegazza, Documents withanticopying means to prevent reproducibility by photocopying, issuedFeb. 9, 1999.

The information may still require more resolution than thescanning-printing process is capable of. See U.S. Pat. No. 5,708,717,Alosia, Digital anti-counterfeiting software method and apparatus,issued Nov. 13, 1998, which discloses a method for combining a sourceimage and a latent image which is visible only when viewed through aspecial decoder lens. This latent image can, for example contain thewords “authentic” repeated several time, or more document-specificinformation such as the personal information in the portrait of a IDcard. However, since the latent image is printed with “sub-pixel”precision, it cannot be easily reproduced. Of course, what is“sub-pixel” today may be easily reproducible tomorrow.

Holograms are inserted on documents such as ID cards and banknotesbecause they are assumed to be easy to detect by the human eye and hardto reproduce with high fidelity. However, while anyone can see whether ahologram is present on a document, an untrained observer will generallybe unable to detect whether the hologram is authentic or a copy.

Common Problems of Invisible Copy Protection Features

A problem that all of the invisible copy protection features have isthat their invisibility makes them completely useless to people who donot have the special instruments needed to read them. Moreover, theinvisibility of the features causes problems with printing and/ordetection. With the watermarks, the need to keep the watermarksinvisible necessarily makes them hard to detect, and that isparticularly the case when wear and tear add extra noise to a document.With invisible ink, both printing and detection are complicated, andthat is also the case with latent images printed with “sub-pixelprecision”.

What is needed is techniques that can reliably determine, at a lowercost, whether a document is an original or a copy, whether the documenthas been altered, or whether it is authorized, that make it possible forthe public to see that a document may be easily authenticated, and thatcan be easily integrated with other techniques for authenticating adocument. It is an object of the inventions disclosed herein to providesuch techniques.

SUMMARY OF THE INVENTION

The object of the invention is attained in one aspect by techniques fordetermining whether an analog form of an object is an original analogform, that is, an analog form made from an original digitalrepresentation instead of by photocopying or scanning an analog form. Ina method employing the techniques, a portion of a digital recording madefrom the analog form is compared with an original digital representationof the portion of the analog form to determine a degree of dissimilaritybetween the recorded portion and the original digital representation ofthe portion and using the degree of dissimilarity to determine whetherthe analog form is an original analog form. A further characteristic ofthe invention is that the dissimilarity that is determined is adissimilarity that is caused by operations involved in making anon-original analog form.

Other characteristics of this aspect are practicing the method in a nodein a network which receives the digital recording from another node inthe network and returning an indication to another node whether theanalog form has been determined to be an original analog form, as wellas practicing the method in a processor to which a digital recordingdevice and an output device are attached. The processor makes thedigital recording from input received from the digital recording deviceand provides an indication whether the analog form has been determinedto be an original analog form to the output device.

In an advantageous embodiment of the invention, the original digitalrepresentation of the portion has a noisy pattern. The original digitalrepresentation may be made using a key and the original digitalrepresentation may have a function in the analog form in addition topermitting determination of whether the analog form is original. Thefunction may be to serve as a barcode or a background image or to carrya message.

Another aspect of the invention is a method of performing anauthenticity check on an analog form. The method compares a digitalrecording of a noisy pattern in the analog form with an original digitalrepresentation of the noisy pattern and the result of the comparison isused to perform the authenticity check. The technique may be used todetermine whether a portion of the noisy pattern has been destroyed inthe analog form. The noisy pattern may further contain a message.

Still another aspect of the invention is a method of hiding a message inan analog form In the method, a digital representation of a visiblenoisy pattern in which the message has been hidden is made and isincluded in the analog form.

Other objects and advantages will be apparent to those skilled in thearts to which the invention pertains upon perusal of the followingDetailed Description and drawing, wherein:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an overview of how a visible authentication pattern (VAP) isgenerated and inserted into a document;

FIG. 2 how a VAP is recorded from a document;

FIG. 3 is a flowchart showing in overview how a VAP can be used inauthentication;

FIG. 4 is an overview of printing and authentication of original andnon-original analog forms;

FIG. 5 shows GUIs for watermark detection and alteration detection;

FIG. 6 is a graph showing correlation between energies in bands offrequencies in an original digital representation of a VAP and a VAPrecorded from a non-original document;

FIG. 7 is a graph showing correlation between energies in bands offrequencies in an original digital representation of a VAP and VAPsrecorded from original documents;

FIG. 8 shows how a message-based key can be used to embed a contentlesswatermark in an image;

FIG. 9 shows a technique for determining whether a particular digitalrepresentation is derived from a digital representation which waswatermarked using a message-based key;

FIG. 10 shows how a VAP may be used to detect alteration of a document;and

FIG. 11 shows how a VAP may be incorporated into a bar code or into alogo.

Reference numbers in the drawing have three or more digits: the tworight-hand digits are reference numbers in the drawing indicated by theremaining digits. Thus, an item with the reference number 203 firstappears as item 203 in FIG. 2.

DETAILED DESCRIPTION

Using the Mere Presence of Watermarks to Authenticate a Document

Generally speaking, authentication techniques for documents whichinvolve watermarks use the watermark to hide some kind of authenticationinformation for the document in a graphical element in the document. Anexample is using the watermark to hide a digest made from the document'scharacter codes, as explained in U.S. Pat. No. 6,243,480, cited above. Adifficulty with techniques that use watermarks to hide authenticationinformation in a graphical element of a document is that wear and tearon the document often renders the watermark unreadable.

U.S. Ser. No. 10/287,206, the parent of the present application,explores ways of obtaining at least some information from unreadablewatermarks and ways of making watermarks more robust in the face oflossy transformations such as those caused by wear and tear on adocument. Among the things that the inventors of U.S. Ser. No.10/287,206 realized in the course of their work were first, that awatermark's mere presence could be used to authenticate a document, andsecond, that the mere presence of a watermark could be used to discoverwhere a document had been altered. The portions of U.S. Ser. No.10/287,206 that deal with these realizations follow.

Watermarks that are Embedded Using Message-Based Keys: FIGS. 8 and 9

The standard application of digital watermarks is to hide a message in adigital representation. One of the uses of such a message is validatingor authenticating the digital representation: the digital representationbeing validated is believed to contain a watermark which contains aparticular message; the watermark is read and its contents are comparedwith the particular message. If they agree, the digital representationis valid or authentic. When the digital representation has undergone alossy transformation, the watermark may become unreadable; thetechniques discussed in U.S. Ser. No. 10/287,206 permit limitedvalidation or authentication in such situations. A general problem withvalidation by means of messages contained in watermarks is thatvalidation often involves long messages such as social security numbersor account numbers, while watermarks containing such long messages areless robust than watermarks containing short messages, and are thereforemore likely to be rendered unreadable by lossy transformations.

A solution to this general problem is based on the observation that forvalidation or authentication purposes, there is no need that thewatermark actually contain the message that forms the basis for thevalidation or authentication; all that is required is that a givenwatermark will be present in a digital representation only if thewatermark was made using the message that forms the basis for thevalidation. In that case, there is no need for the watermark to bereadable; instead, the mere presence of the watermark permits thedigital representation to be validated. Moreover, because it is thewatermark's presence and not its content that shows that the digitalrepresentation is valid or authentic, the watermark's content need donothing more than indicate the watermark's presence and need be nolonger than is required to do that; indeed, the watermark vector for asuch a watermark need only specify the value of a single bit. This inturn makes such watermarks far more robust than watermarks that containthe message that forms the basis for the validation or authentication.

One way of making a watermark whose mere presence in a digitalrepresentation validates or authenticates the digital representation isto use the message to determine the location of the watermark in thedigital representation. This is shown at 801 in FIG. 8. A key function805 (ƒ) is used to make a key 806(K2) from a message 803(m): K2=ƒ(m);where required, the function 805 may use a secret key K1 as well as m tomake the key: K2=ƒ(K1,m). Key 806 is then provided to watermark embedder809 along with a short (minimum 1 bit) watermark vector WM 807 andwatermark embedder 809 embeds a watermark made using watermark vector807 at the locations in watermarked digital representation 813 indicatedby key 806.

The watermark is shown in FIG. 8 by the dotted boxes labeled 807 indigital representation 813. Since message 803 is now no longer containedin the watermark, but instead used to make key 806 and short watermarkvector 807 need only be 1 bit in length, the length of the message hasno effect whatever on the robustness of the watermark. As is well knownin mathematics, there are many functions which can be used to generatekey 806 from message 803 in a fashion such that key 806, and thus thewatermark made with it, is unique to the message. The degree ofuniqueness required may of course vary with the application. In somecases, the function may be an identity function, i.e., the key is themessage itself. An advantage of the technique is that the functiondetermines the length of the watermark key, and thus, the key can bemade as long as is required for a particular application.

FIG. 9 shows at 901 a system that determines whether a digitalrepresentation 903 that is believed to contain a watermark made in themanner just described is authentic. Digital representation 903 containsa set of locations 905 that should contain watermark vector 807 ifdigital representation 903 is in fact derived from digitalrepresentation 813. The locations are at positions which in digitalrepresentation 813 were determined by key 806. The system that is doingthe authentication obtains message 803 and also obtains or is inpossession of key function 805. Key function 805 is applied to message803 to produce key 806 as described above. The system then provides key806 to watermark reader 907, which uses it to find locations 905. When alocation is found, it is output to comparator 909, as shown at 909.Short watermark vector 807 is also in possession of system 901, and itis provided to comparator 911 to compare with the value of each of thelocations 905 in turn. The result 912 of each comparison goes toaggregator 913, where the results are aggregated to produce overallresult 915, which indicates whether the watermark that was embedded indigital representation 813 is present in digital representation 903.Comparator 911 and aggregator 913 can use any of the techniquespreviously discussed with regard to unreadable watermarks for doing thecomparison and the aggregation. As described below for the techniquesused with unreadable watermarks, the pattern of locations 905 that matchthe watermark in digital representation 813 may be used to showlocations at which digital representation 903 has been altered.

In some applications, aggregator 913 will produce a visual result of thecomparison. An example of such a comparison is shown at 501 in FIG. 5.There, the blocks to which the watermark was applied have differentshades depending on the extent to which the presence of the watermarkwas detected. The lighter the block is, the stronger the presence of thewatermark in the block. Because image 501 has undergone lossytransformations, the distribution of blocks with strong watermarks willnot be the same as in the original, but the errors caused by the lossytransformations are random, and consequently, if the image is authentic,all areas which contain the watermark should have roughly the samedistribution of light blocks as shown at 501, This visualizationtechnique can of course be used as well with watermarks in which themessage determines the watermark's contents.

Using Watermarks to Locate Alterations in Digital Documents and AnalogDocuments made from Digital Documents

One way of attacking a digital document or an analog form made from thedigital document is locally modifying an image in the document or formto change its semantic content.

Examples of local modifications can be:

-   -   modifying the plate number on the image of a car captured by a        DVR on the scene of an accident/crime; or    -   modifying areas of the portrait on an ID card; or    -   replacing the portrait on an ID document.

If the document or form is watermarked, the counterfeiter's goal is tochange the semantic content of the digital document or form withoutrendering the watermark incorrect or unreadable. In general, when awatermark is robust enough to be readable, it will not be difficult forthe counterfeiter to make small changes in the document or form withoutrendering the watermark incorrect or unreadable. On the other hand, thevery robustness of the watermark makes it useful for detecting andtracking alterations.

In order to use a watermark to locate an alteration, one need only knowthe locations at which the watermark is expected to be and its watermarkvector. Since the technique does not require that the watermark have anyparticular content, the watermark vector need only be a single bit. Oncethe detector knows the watermark locations and the watermark vector, thedetector can use the watermark vector w′ which is a replica of theoriginal watermark's watermark vector w and compare w″ with thewatermark w″ in the questionable content. Differences between w′ and w″may show whether the digital document or analog form that is the sourceof the questionable content has been modified and if so, which portionswere modified.

In more detail, the detector compares the watermark vector w″ in eachsubpart (termed herein a block) of the digital document or analog formwith vector w′. The comparison indicates whether each block of thedocument or form holds the correct watermark information. In a digitaldocument, if there has been no alteration, most blocks will contain thecorrect watermark information. With analog forms, the print-and-scanprocess deteriorates the watermark, and consequently, not all blockswill hold the correct watermark information (e.g. there can be in theorder of 20% to 40% errors). These printing and scanning errors aregenerally of random nature and therefore can be expected to bedistributed more or less uniformly on the analog form. Thus, if theimage has been locally altered and has thereby lost its watermark in thealtered areas, the watermark detector will respond to the altered areasin the same way that it responds to areas that are not watermarked. Indoing so, the watermark detector detects the alteration. The techniquecan also be used to show the strength of the watermark in each area ofthe image.

The replica watermark vector used to detect alterations or watermarkstrength may come from any source. Examples include the original image,a watermark vector from the questionable content that has beensuccessfully read, or a watermark vector which has been generated anewfrom the message. Adaptive embedding and detection may be used toincrease the effectiveness of detecting alterations. For example, areasof the content that need special protection against change may receivewatermarking of a greater strength than other areas of the content, andthe greater strength of the watermarking in these areas may be takeninto account when the watermarks are analyzed as described above. Ofcourse, the technique as used to show the strength of the watermark ineach area of the image may be employed to aid in the design of masks foradaptive embedding and detection.

Different techniques inspired by statistics, signal processing orpattern recognition can be applied to automatically detect areas thatcontain an abnormally large number of blocks that hold incorrectinformation (or no information at all). For example, one techniqueinspired from pattern recognition is to determining connections ofincorrect blocks, and extract those connections that are higher than athreshold. Another technique would be to determine in all areas of sizeN×N of the analog form whether there are more than P incorrect blocks.Yet another technique from signal processing is to assign positivevalues to correct blocks and negative values to incorrect blocks andthen low-pass filter the resulting matrix. The areas of the filteredmatrix in which values are under a threshold are detected as having beenaltered. Finally, statistics can be applied in all approaches tocharacterize areas of the images that are not altered and those that arealtered, and to determine detection parameters relatively to the user'sexpectation (e.g. minimum size of altered areas, probability of falsealarm/rejection, etc). It is also possible to display to the user animage with the incorrect and correct blocks in different colors, toallow human interpretation of the data.

FIG. 5 shows the effect of alterations on watermark strength and alsoprovides an example of a graphical way of showing altered areas. Here,image 501 was modified after it was watermarked by replacing the face inwith another face which was not watermarked in the way that the face inimage 501 was watermarked. The result of the modification is image 502.When image 502 is compared with image 501, it will be seen that thefacial area of image 502 is darker than the facial area of image 501.This in turn shows that the blocks in the facial area of image 502 arefar more weakly watermarked than the blocks in the facial area of image501. The weak watermark in the facial area of image 502 is of course adirect consequence of the modification. When a filter is applied thathighlights areas with many weak blocks, the result is image 503, inwhich modified area 505 clearly stands out.

Extensions of the Technique

-   -   Detecting more than one altered area    -   Using external modules (e.g. face recognition), to focus        detection of alteration on the most semantically significant        areas (e.g. the eyes in an ID photo)    -   Multiple scanning of physical document to cancel out scanning        variability.

If the watermark is unreadable, the alteration detection may be used foranalyzing the reasons for its unreadability.

Visible Authentication Patterns

The foregoing realizations were followed by the realization that led tothe present invention: when a watermark's mere presence is being used todetermine authenticity of an analog form, the watermark is being used asa contentless pattern. Since the pattern has no content, there is nolonger any need for it to be invisible; instead, it can be added to thedocument as a visible element. In the following, visible patterns thatare used for authentication are termed visible authentication patternsor VAPs. Because the VAP is visible, it is far easier to detect than isa watermark. It is, however, still able to perform all of theauthentication functions of invisible watermarks and in addition letsconsumers of the document know that the document's authenticity isprotected.

Terminology

The following terminology will be used in the Detailed Description toclarify the relationships between digital representations and analogforms.

A digital representation of an object is a form of the object in whichthe object can be stored in and manipulated by a digital processingsystem. Objects may be or include as components documents, images,audio, video, or any other medium of which a digital representation canbe made.

an analog form of a digital representation is the form of an object orcomponent that results when the digital representation is output to ananalog device such as a display, printer, or loudspeaker.

a digital recording of an analog form is a digital representation madefrom the analog form. The manner in which the digital recording is madedepends upon the medium; for example, for a documents or an image,digital recording is done by digitizing an image made from an

analog form of the document or image. an original digital representationis a digital representation made or copied by someone authorized to doso; an original analog form is one made from an original digitalrepresentation.

a non-original digital representation is one that is made by digitallyrecording an analog form without authorization; a non-original analogform is made from a non-original digital representation or byphotocopying an analog form.

a document will be given the special meaning of any analog form which isproduced by a printing process, including documents in the more usualsense of the word, labels, packaging, and objects that are themselvesimprinted. To the extent that reasonable analogies can be made,everything in the following that is said about documents may be appliedalso to other media. For example, an audio analog form may include anaudible authentication pattern that is the audio equivalent of the VAP.

Making a Visible Authentication Pattern: FIG. 1

The paradox of the visible authentication pattern is that while thepattern is visible, a possible counterfeiter must not be able to modifythe pattern so that it will authenticate a document that is notauthentic. This end is achieved in a preferred embodiment by making thepattern noisy, i.e., a large part of the value of the pixels making upthe pattern is apparently randomly determined. Because the pattern isnoisy, it is impossible to tell what values the pixels making up thedigital representation of the pattern should have without access to theoriginal digital representation of the pattern. On the other hand, giventhe original digital representation of a VAP, one can compare a digitalrecording of a VAP from a document with the VAP's original digitalrepresentation, determine how the recorded VAP has been altered withregard to the VAP's original digital representation, and can determinefrom the differences how the document in question has been altered. Aswill be seen in more detail in the following, alterations that can bedetected include those involved in making non-original documents andthose involved in altering information in a document.

FIG. 1 shows one way of making a visible authentication pattern andinserting it into a document. There are three steps:

-   -   generating a digital representation of the pattern, shown at        101;    -   an optional step of adding a visible logo or legend to the        authentication pattern, shown at 107; and    -   inserting the authentication pattern into the document, shown at        113.

The original digital representation of the pattern 105 can be generatedin any way which produces a result in which the pattern's pixels appearto have values with a strong random component. The digitalrepresentation of pattern 105 may be a gray-scale pattern, or it mayemploy colored pixels. It is particularly useful to employ a key togenerate the pattern; the key 103 is used as a seed for a pseudo-randomnumber generator which produces the sequence of values which are givento the pixels in the pattern. Uses of the key will be explained indetail later. The original digital representation of pattern 105 mayalso include components which aid in locating the pattern in a digitalrepresentation made by scanning a document that contains pattern 105. Inpattern 105, black border 106 performs this function.

A visible logo or legend 109 can be added to the original digitalrepresentation of pattern 105 to make the original digitalrepresentation of pattern 111 without compromising pattern 105'snoisiness because only a part of the value of the pixels making up thepattern need be randomly determined. Thus, the logo or legend can besuperimposed on pattern 105 by manipulating the values of the pixelsmaking the logo or legend in a way that preserves their randomness whilecausing the logo or legend to appear. For example, if pattern 105 is agray scale pattern, the legend or logo can be made by making the pixelsof the legend or logo uniformly darker or lighter relative to theiroriginal random values. The technique is similar to adding a visiblewatermark to an image, except that it preserves the noisiness of pattern105.

Once the original digital representation of pattern 111 has been made,it is inserted into the original digital representation of the document115, as shown at 113. When document 117 is printed from original digitalrepresentation 115, document 117 includes printed visible authenticationpattern 119. Of course, the document may be printed onto a substratethat already has printed material on it. Thus, pattern 119 may be addedto a preprinted substrate.

Using a Visible Authentication Pattern to Authenticate a Document: FIGS.2 and 3

When a document that contains a printed VAP 119 is authenticated, thefollowing happens:

-   -   a printed VAP 119 is detected in the document.    -   a digital recording of the detected printed VAP 119 is made.    -   the digital recording of the printed VAP is compared with the        original digital representation of the VAP; and    -   authenticity is determined on the basis of the comparison.

The manner in which the digital recording of the printed VAP is comparedwith the original digital representation of the VAP depends on the kindof authentication being done; further, the authentication of a givendocument may involve several different kinds of comparisons being madebetween the digital recording and the original digital representation.For example, a digital recording of a visible authentication pattern onthe amount field of a check may first be compared with the originaldigital representation to determine whether the check is a counterfeitand second to determine whether the amount in the amount field has beenaltered.

FIG. 2 shows detecting the printed VAP and making a digital recording ofthe VAP in a preferred embodiment. Both are done using the “Scanread”application program available from MediaSec Technologies. Otherapplications that detect a portion of a document and make a digitalrecording of it may also be employed. Scanread 201 uses black border 106to detect the presence of visible authentication pattern 119 in printeddocument 117 and then makes digital recording 203 of visibleauthentication pattern 119. FIG. 3 shows a general flowchart 301 of aprogram that uses digital recording 203 and original digitalrepresentation 111 of VAP 119 to determine authenticity. Originaldigital representation 111 of the VAP may be the original itself, a copyof the original, or a new original digital representation 111 made inexactly the same way as the first original digital representation.Original digital representations obtained by any of these methods are ofcourse exactly equivalent, and which method is used is a matter ofimplementation issues such as the cost of storage for the originaldigital representation of the VAP, the cost of transmitting the originaldigital representation of the VAP across a network, and the cost ofgenerating the original digital representation each time it is required.

Beginning at 303, features of digital recording 203 and original digitalrepresentation 111 are compared at 305; what features are compared andhow they are compared depends on the kind of authentication being done.If the differences between digital recording 203 and original digitalrepresentation 111 exceed a threshold (307), there is an authenticationproblem and branch 309 is taken. The threshold will also depend on thekind of authentication being done. In branch 309, the existence of aproblem is indicated to the application program that is doing theauthentication at 311. Where it is useful, the program may also provideinformation about the comparison (315); again, the kind of informationand the manner in which it is provided will depend on the kind ofauthentication. For example, if the amount in the amount field appearsto have been altered, the program may display an image that shows whichof the pixels of the original digital representation appear to have beenaltered in the digital recording of the visible authentication pattern.If the differences do not exceed the threshold, branch 317 is taken.There, the fact that no authentication problem has been detected isindicated to the application program that is doing the authentication.Both branches and the program terminate at 321.

Using Visible Authentication Patterns to Distinguish an OriginalDocument from a Non-Original Document: FIGS. 4, 5,

One way a visible authentication pattern can be used to authenticate adocument is by determining whether a document is an original, i.e., wasprinted from an original digital representation or is a non-original,i.e., was photocopied from document or was printed from a non-originaldigital representation, that is, a digital representation that was madefrom an unauthorized digital recording of a document. The reason avisible authentication pattern can be used in this way is that printinga document from its digital representation and making a digitalrepresentation of a document from a digital recording of it orphotocopying a document always result in losses of information in thevisible authentication pattern, regardless of how precise the printing,digital recording, or photocopying processes are; consequently, one candetermine by comparing an original digital representation of a visibleauthentication pattern with a digital representation made by recordingthe visible authentication pattern from a document whether the documentis an original or a non-original. In the case of an original document,the visible authentication pattern will have been printed once anddigitally recorded once; in the case of a non-original document, thevisible authentication pattern will have been printed and digitallyrecorded once to produce the original document from which thenon-original document was made, and then, depending on how thenon-original document was made, either photocopied or again printed anddigitally recorded, resulting in a greater loss of information in thenon-original document's visible authentication pattern than in theoriginal document's visible authentication pattern.

The basic technique is shown in detail in FIG. 4. At 401 is shown howauthentication using a visible authentication pattern works with anoriginal document. Original digital representation 403 of the documentcontains an original visible authentication pattern (ovap)

405. Original digital representation 403 is then printed at 407 toproduce original analog form 409. The printing operation causes loss1 inoriginal analog visible authentication pattern (oavap) 411 in analogform 409. When authenticator 421 authenticates analog form 409, it makesa digital recording of oavap 411, resulting in loss2. The recordingappears as roavap 415. Authenticator 421 then employs comparator 417 tocompare ovap 406 with roavap 415. The difference between them is the sumof loss1 and loss2. That will be true when any otherwise undamagedroavap 415 is compared with ovap 405, and a difference of that size is adependable indication that analog form 409 is indeed an original analogform.

At 420 may be seen how authentication works with a non-originaldocument. The difference between the original document and thenon-original document is that the non-original document is not printeddirectly from original digital representation 403 of the document, butinstead from a non-original digital representation 423 of the documentwhich has been made by digitally recording an original document 409(422). As a result of the digital recording, the non-original visibleauthentication pattern 425 in digital representation 423 has suffered anadditional loss of information which appears in FIG. 4 as loss3. Whennon-original analog form 429 is printed (427) from digitalrepresentation 423, another loss occurs in non-original analog visualauthentication pattern 431, indicated as loss4. When non-original analogform 429 is authenticated by authenticator 421 as described above andmoavap 435 made from noavap 431 is compared with ovap 405, the effect ofloss3 and loss4 will show up as a greater difference between ovap 405and movap 435 than there was between ovap 405 and roavap 415. Sincenoavap 431 in a non-original analog form 429 will always undergo theadditional losses 3 and 4, the larger difference is a dependableindicator of a non-original document.

Non-original analog form 429 can of course be produced by anyphotocopying process as well as by the process of recording the originalanalog form (422) to make a non-original digital representation 423 andthen printing (427) digital representation 423 to produce non-originalanalog form 429. The process of acquiring the image of original analogform 409 and then printing non-original analog form 429 from the imagecauses additional losses like those of losses 3 and 4, and consequently,rnoavap 435 produced in this fashion will still be less similar to ovap405 than roavap 425.

Of course, if non-original digital representation 423 is itself madefrom a non-original digital representation, movap 435 will include theadditional losses resulting from the photocopying or printing anddigital recording of that non-original digital representation as well.Obviously, if loss1 and loss2 were fixed values, the detector couldalways determine correctly whether the document is original ornon-original. However, in general some variation will occur for eachloss, for instance some originals could be printed with a better quality(fidelity) than others. It seems then that a statistical approach todetection should be employed.

Details of a Preferred Embodiment of the Technique for DistinguishingBetween an Original and a Non-Original Document: FIGS. 6 and 7

An authentication technique is only as good as its reliability. The keyto minimizing the probability of detection errors is the method formeasuring how “different” a visual authentication pattern recorded froma document is from the original digital representation of the visualauthentication pattern. The measurement method chosen must be based onproperties of the VAP that are affected by the process of making anon-original document and must clearly distinguish an original from anon-original document.

Our approach is to consider the photocopying, recording, and printingprocesses as filters, more specifically as low-pass filters. Hence, highfrequencies will be more attenuated than low frequencies by the printingand recording processes, and will lose more information at eachrecord-and-print or photocopying step. For low frequencies in which arecord and print or photocopying process preserves nearly all energy, aVAP in a non-original document may not have significantly lessinformation the VAP in an original document. The very high frequenciesmay also not be helpful, since most of the energy at these frequenciesin the VAP is lost the first time the VAP is printed. Consequently, eventhe VAPs of original documents contain very little information fromthose frequencies. Therefore, one must make an appropriate selectionand/or weighting, of the frequencies used by the detector. The selectionof frequencies for comparison, as well as the selection of a thresholdfor determining whether a document is original or non-original istypically done by training the comparison software on VAPs from originaldocuments.

It should be pointed out here that the technique described above doesnot require a special visual authentication pattern. Instead, the entiredocument or a part of it can be used as the pattern. However, becausemany documents may not contain information at the energy levelsnecessary to determine whether a document is an original or a copy, itis better to use a visual authentication pattern which containsinformation at the proper energy levels. In the following, such visualauthentication patterns will be termed copy detection patterns, or CDPs.The information in a CDP is distributed in appropriate frequencies. In apreferred embodiment, the original digital representation of the CDP isgenerated pseudo-randomly by a key, and consequently a program that hasaccess to the key can create a new copy of the original digitalrepresentation of the CDP at any time. This key can be kept secret orrevealed only to trusted parties. The copy detection pattern is insertedor printed on the document to be secured. In a preferred embodiment,analysis of a copy detection pattern from a document is done bydigitally recording the document's CDP, using the key to generate a newcopy of the original digital representation of the CDP, and comparingthe recorded CDP with the original digital representation of the CDP. Inother embodiments, the recorded CDP may simply be compared with apreexisting copy of the original digital representation of the CDP.

Algorithms Used in the Technique

This section describes the algorithms used for (1) generating anoriginal digital representation of a CDP; (2) detecting and extracting aCDP from a document; (3) comparing the original digital representationof a CDP with a recorded CDP; and (4) determining whether a CDP isoriginal or non-original. The manner in which the CDPs are compared inalgorithm (4) and the thresholds for determining whether a CDP isoriginal or non-original are determined by a training process in whichalgorithm (3) is used to gather training data.

Generating the Original Digital Representation of the CDP

The function make_pattern is used to create a digital representation(pattern_img) of a copy detection pattern that may be identified with asource of the digital representation from which an original document ismade. make_pattern generates a noisy gray-scale or color pattern. Ablack border may also be added to the pattern to facilitate itsdetection in the document. The CDP may optionally also display a logo.The logo will typically affect the lowest frequency bands, and itsimpact on detection will be therefore limited. Typical values are givenin the explanation of parameters.pattern_(—) img=make_pattern(type, height, width, key, filename, border,logo_(—) img, logo_weight).Parameters for Pattern GenerationRequired:

-   1. Type: type of generated random number values, e.g. ‘randn’    (gaussian N(0,1)), ‘rand’ (equiprobable distribution), randint    (binary +1 or −1 distribution), or MD5, SHA algorithms (0-255    integer number). The random number values are then used to compose a    grayscale or color image.-   2. Height: height of pattern in pixels (e.g. 104).-   3. Width: width of pattern in pixels (e.g. 304).-   4. Key: integer-valued secret key or password used as a seed for the    random number generator.    Optional:-   5. Filename: name of the file in which the pattern image is saved.-   6. Registration mark (e.g. black border added on the sides of the    pattern image, dots added at the four corners of the pattern image).-   7. Logo_img: image to be used as background logo, automatically    scaled to the dimension of the pattern image.-   8. Logo_weight: value between 0 and 1 to weight the energy of the    logo image (e.g. 0.2), which is superimposed on the pattern image.    An Example of the Use of Pattern Generation Algorithm:-   1. Generate pattern in a specific domain (e.g. DCT luminance or    spatial in color RGB mode):    pattern=generate_pattern(type, height, width, key);-   2. Transform the pattern to the spatial domain if the domain in the    Step 1 is not spatial (e.g. inverse DCT):    pattern_(—) img=transform (pattern);-   3. If required round up pixel values p to integer values 0<p<255.-   4. Combine logo with pattern, for example, the mixing following    function can be:    pattern_(—) img=(1−logo_weight)*pattern_(—) img+logo_weight*logo_(—)    img;-   5. Add registration mark (e.g. black border).-   6. Dump image.

A pattern image may consist of multiple components/channels such as Red,Blue, Green, or YUV, which can be produced as described in Steps 1 and 2above.

To combine a CDP with logo or background image, various mixing functionscan be adopted. For example when the CDP is merged with a barcode(image), the CDP replaces only the black area of barcode and leave thewhite areas untouched.

Any shape (such as circle, oval) of the pattern image can be generated.A simple approach is to use a “shape mask” which defines an arbitraryshape represented by a two-dimensional array consisting of “1” and “0”.Any shape can be created by applying the “shape mask” to the rectanglepattern image.

Detecting and Extracting the VAP from a Document

In this implementation, a digital recording of the document beingauthenticated is made and the black border on the VAP is used to locatethe VAP in the digital recording. The black border results in a strongvariation of luminance in the transition region, which is easilydetectable. Other techniques for determining the location of the VAP maybe used as well (egg existing features in the documents, black dots,etc.). Once the VAP has been detected, a digital representation is madeof it which is comparable with the original digital representation ofthe VAP. This digital representation is the recorded VAP.

The original digital representation of the VAP and the recorded VAP arecompared using the following function, that measures an index thatindicates how “close” the recorded VAP is from the original digitalrepresentation of the VAP. The original digital representation of theVAP can be stored in the memory of the detector, or can be re-generatedif the parameters used to create the original digital representation andthe function make_pattern( . . . ) are available to the detector. Theoptional parameters used when combining the pattern with a logo may notbe required, because the logo generally affects the properties of thepattern only slightly. The function for doing the comparison isanalyze_pattern, which returns Results, and may take differentparameters depending on the scenario that is actually applied:Results=analyze_pattern (type, height, width, key, . . . , test_(—)img);ORResults=analyze_pattern (orig_(—) imgtest_(—) img);Parameters and Output:

-   1. type, height, width and key: these are as explained for pattern    generation.-   2. test_img: test pattern image extracted from the document.-   3. orig_img: original digital representation of the pattern-   4. Results: contain all the results of the analysis For example, it    may include different measures of correlation or statistics,    computed for different elements of the images eg different    frequencies, different areas, different color channels, etc.)

The following example shows the steps of the algorithm the originaldigital pattern is regenerated and the subfunctions required for thealgorithm:

-   1. (Optional) Remove the black border from the test CDP-   2. Transform the test pattern image into the domain in which it was    originally generated, for example, 8×8 block DCT:    test_pattern=transform (test_img);-   3. Regenerate the original CDP:    pattern=make_pattern(type, height, width, key);-   4. (Optional) Locally synchronize the test CDP with the original CDP    as described below. (Optional) Apply certain image filters (such as    sharpening) to the test CDP in order to produce a better correlation    with the original CDP.-   5. If required, convert the original CDP and test CDP into the    domain where the comparison is to be made (eg 8×8 block DCT). Note    that the comparison can be made in more than one domain, for example    in both the spatial and frequency domains.-   6. Compute several measurements of similarity between the original    CDP and the test CDP for each channel in the transformed domain. For    example, if patterns are generated and recorded in the color RGB    domain, and the analysis is made in the 8×8 block DCT domain. Then    there are 192 (i.e. 8×8×3) combinations by means of which the two    patterns can be compared, and hence 192 measurements of similarity    can be performed. The measure of similarity can itself be computed    in several ways, for example by binning values and keeping only the    one where there is a higher correlation, in order to exclude areas    of the test CDP that may have been corrupted.-   7. Collect and combine all similarity measures or measures based on    other image features, in order to measure one or more indices of    quality or of the “closeness” of the test CDP to the original CDP.    The combination function can be any function that combines the    different inputs, for example a function that combines similarity    measures by assigning more weight or importance to features that are    better discriminants between the original CDP and the test CDP.

As already explained above, a duplication process will always degradethe original CDP, and in general it is expected that the differentmeasures of closeness or quality will be lower for a CDP that isrecorded from an analog form. However, due to statistical variations, anadequate selection and combination of the different measures can be moreeffective in determining whether a test CDP is recorded from an originalanalog form or from a non-original analog form.

FIG. 6 shows the correlation (shown at 605) between the energies of thefrequencies in the original CDP and the test CDP from the document beingauthenticated for thirty bands of frequencies (shown at 603). Asexpected, the correlation between the energies is highest in the lowfrequency bands from which little information is lost in the copyingprocess and lowest in the high frequency bands where even a singleprinting operation causes the loss of most of the information. If thecorrelations are substantially lower in the middle frequency bands thanthey would on average be for CDPs from original documents, the CDP isnot an original, and therefore neither is the document beingauthenticated. That is the case for the plot of FIG. 6, which thus showsthat the document being authenticated is not an original.

Other image features can also be considered when the correlation valuesby themselves are not sufficient to determine whether a document is anoriginal analog form or a non-original analog form. Additional imagefeatures which can be used for producing correlation values between theoriginal CDP and the test CDP include:

-   -   color histogram    -   edge, line and outlines    -   frequencies in other domains (such as Fourier and Wavelet        domains)    -   brightness and contrast        Detecting Whether a CDP is From an Original or a Non-Original        Document

The function detect_Pattern analyzes the results returned byanalyze_pattern and returns the value Output, which indicates whether aCDP is from an original document or a non-original document.Output=detect_pattern (Results, Parameters)

Results: can be a scalar value or a vector, the output of the functionanalyze_Pattern.

Parameters: values required to adjust the behavior of the detectionfunction, which may depend on the requirements of the application andthe conditions under which it performs detection.

Output: different output values are possible. In its simplest form,Output may take three values: ORIGINAL, NON-ORIGINAL, orPROCESSING-ERROR. The last output may occur when the pattern is badlyrecorded. Output may return more detailed information, for example,NON-ORIGINAL can further indicate how the test pattern from thenon-original document was produced (eg duplication, photocopy,regeneration, etc.). Output can further provide indexes of quality orcloseness.

Here is an example of the algorithm for a simple detection function:

-   1. Combine the various Results values returned by analyze_pattern to    obtain a scalar value S. One way of doing this would be to make S by    summing the returned Results.-   2. If S>T1 then output is ORIGINAL, else if S>T2 then output is    NON_ORIGINAL, else the output is PROCESSING ERROR.

Here T1, and T2 are two scalar parameters typically obtained via atraining process, with typically T1>T2.

Local Resynchronization of the CDP from the Document with the OriginalCDP

In order to compare the CDP recorded from the document with the originalCDP, the recorded CDP must be synchronized with the original CDP. Oneway to do this is to use synchronization points in the recorded CDP, forexample, black border 601, to synchronize the original. Once the CDPsare synchronized, the comparison between them is done pixel-by-pixel orblock by block.

When there have been errors in printing the CDP in the document or inthe digital recording of the CDP from the document, the CDPs cannot beperfectly synchronized by this method. For example, there might be lessthan a pixel shift between the original CDP and the one recorded fromthe document. Furthermore, the shift may vary along the pattern: in somecases the upper part of the recorded CDP may be shifted downwardcompared to the original CDP and the lower part be shifted upward (orvice-versa, of course). These shifts may be very hard to notice, may notoccur consistently, and may vary locally in the recorded pattern. Theyare generally caused by slight instabilities in the printer, but canalso be caused by similar instabilities in the recording device.

These unpredictable sub-pixel shifts may reduce the detector'sperformance: because of these misalignments, some CDPs from originaldocuments may be detected as being from non-original documents. Onemethod of handling these “pathological” CDPs from original documents,and in general of improving the stability of the CDP detection is tolocally resynchronize the CDPs in order to correct the localmisalignments. There are several ways to perform localresynchronization, but the general idea is to use the recorded CDPitself for local resynchronization.

One way to perform local resynchronization is to divide the original CDPinto blocks (non-overlapping blocks are preferred, but the blocks couldalso overlap) and find which block of the recorded CDP has the closestmatch with a given block of the original CDP. If there were nomisalignment, the block of the recorded CDP that most closely matchedthe given block would be at the same position in the recorded CPD thatthe given block had in the original CDP: for example, the best match forthe 10×10 block with starting position (80,80) and ending position(89,89) of the original CDP would be the corresponding block (80,80) to(89,89) of the recorded CDP. However, if there is a misalignment, thebest match could as well be with block (81,80) to (90,89) (shift of onepixel to the right). If that is the case, then the recorded pattern willhave the block (81,80) to (90,89) shifted 1 pixel to the left, toposition (80,80) to (89,89). The same idea can be applied to each blockin the recorded CDP, to produce a “locally resynchronized” CDP.

Local resynchronization requires a couple of parameters and functions.First, we must define a measure of distance between each block of theoriginal CDP and a block of same dimensions of the recorded CDP. Aconvenient measure for this purpose is the standard correlationcoefficient. It is also necessary to set the dimensions of the blocksinto which the original CDP is divided: typically a block of dimension8×8 or 16×16 can be used, but in general blocks of size N×M can be used.As mentioned earlier, blocks can be overlapping, in which case theamount of overlap between successive blocks needs to be defined. Anotherparameter to set is the search range or search area: starting frommatching positions, how far should the algorithm look for a matchingblock? This is set with a parameter n, where for block starting atposition (xy) of the original CDP, all blocks with position(x+/−i,y+/−i), 0<i<n, are tested.

It is also possible to scale the digital and recorded CDPs before doinglocal resynchronization: this allows a finer grain match. For example,by scaling the two CDPs by 2, we can recover half pixel shifts. Andfinally, the synchronization algorithm can be applied iteratively on theresynchronized CDP until no further improvement is found.

Once the resynchronization is performed, an arbitrary measure ofsimilarity/distance between the resynchronized recorded CDP and theoriginal CDP can be performed. A simple correlation, or a localfrequency analysis can be performed, perhaps with parameters based on atraining set. These measures, which typically make an average of certainquantities on the whole CDP, may however not always be robust againstsome local damage to the scanned CDP that may occur in certainapplications. For example, in some cases one area of the CDP may havebeen badly printed, or may have been damaged by scratches, writing, orwater. In other cases the scanning device may have inserted distortioninto the scanned CDP; that problem typically occurs with feed-throughdevices when the document is not correctly inserted. To make the CDPmore robust against these kinds of distortion, more robust measures ofsimilarities may be used: one such measure is the median localcorrelation coefficient, where a correlation coefficient is computed foreach block of the CDP, and the median of all local correlationcoefficients is computed. Here, computing a median instead of an averagemakes the detector significantly more robust to local alterations. Tocope with a larger amount of corrupted areas in the CDP, it is alsopossible to compute the average of only the 20% best local correlationcoefficients, which can be assumed to be non-corrupted. In oneimplementation, this procedure of computing is this sort of “biased”average is applied separately to each frequency channel, and optionallyto different color channels. Of course, the foregoing synchronizationtechniques can be applied not just with CDP's, but with any recordedvisible authentication pattern that needs to be synchronized with anoriginal visual authentication pattern.

Applications of CDPs

CDPs can be used in any situation where it is useful to distinguish anoriginal document from a non-original document. A CDP may be printed byany process which prints the CDP with sufficient fidelity so that adigital recording of the CDP is comparable with the original digitalrepresentation of the CDP. The pattern may be particularly adapted todetect non-original documents made by particular photocopying, scanning,or printing techniques. Particular uses of CDPs include:

-   1. Printing a CDP on packaging for brand protection-   2. Printing a CDP on checks and currency for copy detection-   3. Printing a CDP on valuable documents including certificate,    contracts, and the like for verifying whether the document is the    original or a copy.-   4. Printing a CDP on holograms-   5. Printing a CDP on labels on valuable goods such as    aviation/automobile parts or pharmaceuticals.

More generally, a CDP may be used in any application where it isdesirable to be able to determine what processes have been applied to adocument. The pattern may of course be varied as required to best detectthe processes of interest.

CDP can also be used for the following applications:

-   1. Benchmarking of Printing Quality

When reading the CDP, a quality index of the digital recording of theCDP is computed. This quality index will vary on printing quality,paper/substrate quality, or digitization/scanning (device) quality. TheCDP quality index can then be used to quantify the quality of a certainprinting process, a certain substrate or a certain scanner.

-   2. Quality Control

In the same vein, a CDP reader can be used in a printing productionprocess for automatic quality control. The advantages of the CDP overmanual inspection is that it gives an automated, objective, and precisemeasure of quality.

-   3. Tracing

The CDP has a structure and characteristics that is associated with theprinter, paper, camera, and usage and wearing. In principle, analysis ofthe CDP can determine the general “history” of the document: how it wasprinted and what “wear and tear” it has suffered.

Using Visible Authentication Patterns to Detect Alterations inDocuments: FIG. 10

Certain classes of documents are always “modified” after they areprinted. One common example of this is a check that is printed withblank fields that are filled in when the check is written. A problemwith documents belonging to all of these classes is that what is placedin the filled-in fields may be altered later. Thus, even though thecheck itself is authentic, the semantic values of what was written inthe blank fields may be changed. For example, a payee of a check canmodify the amount on a check that is addressed to him (e.g. from “onehundred” to “nine hundred”), in a way that is difficult for a teller tonotice. This kind of problem is hard to solve because the forgers do notactually create counterfeit documents; instead, they alter the semanticvalue of authentic documents. The problem is made harder by the factthat the filled out authentic document already contains legalmodifications. The problem is, how are the legal modifications to thedocument to be distinguished from later illegal modifications.

One of the solutions to this problem is forensic examination. If theteller suspects that the check has been modified, he can bring it toanother authority for further examination. However this task is manual,costly, and time-consuming and it is clearly not possible to apply itsystematically to every document or check. Often, the counterfeiterforges a check by first erasing a part of the writing. For example, tomodify the amount from “two hundred” to “nine hundred”, he will probablyerase the “two” and modify it to “nine”. To erase handwriting, he willoften use chemical products. Another possibility is to scrape theoriginal amount from the check, repaint the background, and then writein the new amount.

Visible authentication patterns can be used to detect these illegalmodifications. The general idea is to print a VAP in each of the areasof the document where we may want to detect illegal modifications. Thelegal modifications are then made by writing on the VAP. The precise,unique and uncopyable VAP structure can be used later on to detectmodifications and to determine if the modifications are acceptable. Theidea is that both writing on a VAP and erasing something written on aVAP produce detectable modifications of the VAP. Writing on the VAPdestroys the pattern, as does scraping writing off of the VAP orapplying a chemical erasing agent to the VAP. A VAP {that is used inthis fashion is termed in the following a modification detectionpattern, or MDP.

How a MDP may be used to detect illegal modifications can be summarizedas follows:

-   -   insert an MDP in each area of the document which needs to be        protected against unauthorized modifications.    -   When verifying the authenticity of the document, first record an        image of each of the MDPs in the document.    -   for each recorded MDP, compare the recorded MDP with the        original digital representation of the MDP to detect areas where        the MDP has been damaged.

The results of the comparison of the recorded MDP with the originaldigital representation of the MDP can be used in a number of ways:

-   -   Display the results of the comparison with the damaged areas        highlighted to a decision maker. This will show both the areas        that contain writing and the erased areas.    -   Display the results of the comparison with non-written damaged        areas highlighted to the decision maker.    -   Compare the size of the damaged area with the size of the area        that has been written on, and if the difference is above a        threshold, treat the field has having been modified.

FIG. 10 shows how a MDP can be used to detect modifications. At 1001 isshown a MDP 1002 that is used in an amount field for a document. Asbefore, MDP 1002 is surrounded by black border 106. As shown at 1003,the amount 250 has been written into MDP 1002. At 1005 may be seen how aforger has modified the amount $250 to the amount $950 by erasing the“tail” of the 2 and adding a loop to make it into the number 9. To coverup the erasure, the forger has imitated the pattern of the MDP. Theimitation is still visible in 1005, but even as shown, it is good enoughto get by a harried teller and a skilled forger can easily make theimitation better.

The problem for the forger is that the erasure has destroyed the MDP. Byscanning the MDP and locally analyzing it, it is possible to detect withhigh accuracy which part of the MDP has changed from the original.Erasures can be detected by finding areas in the MDP which neithercontain neither text nor the original pattern. This is shown at 1009.Text areas are easy to find because they are typically color-uniformedand darker than the MDP. All that then need be done to find the erasedareas is to compare the areas of the recorded MDP that do not containtext with the original digital representation of the MDP. The erasedareas show up as parts of the recorded MDP that do not match theoriginal digital representation, as shown at 1011. In a preferredembodiment, such non-matching parts appear in red.

A few more details on the algorithm for using an MDP to detectalteration of a document:

-   -   Making MDPs: A MDP may be made in any way that a VAP is made,        but then the pixel values are increased to make the MDP brighter        (otherwise, the text written on the MDP could not be easily        distinguished from the MDP).    -   Use registration marks (e.g. black border or corner marks) to        extract the recorded MDP from the document.    -   Detect text areas: A low-pass filter is applied to the recorded        MDP, and pixels with values under a threshold are considered to        be part of the text and legal modifications.    -   Detect modifications of the MDP: after local resynchronization        is applied, a correlation coefficient is computed for each block        of the MDP. As shown in 1009 one can see that the areas of the        text and the areas of the illegal modification were altered.    -   By excluding the legal modifications (at 1003) from image 1001,        several algorithms can be applied to detect the illegal        modifications. One possible way is to first classify areas into        modified or non-modified (by thresholding the local        correlation), then apply a noise processing algorithm or        low-pass filter that removes individual or non-significant        modified areas. Region detection algorithms can also be applied        to find significant modified regions. The result is displayed in        1009: the non-allowed modifications are displayed in red, while        the allowed one (on the text) areas displayed in green.    -   Depending on the amount of non-allowed modifications, a decision        can optionally be taken on the authenticity of the document to        which the MDP belongs.        Implementation Details of the VAP        Form of the VAP in the Document

All that is required for using a VAP to detect alterations in an analogform is that there be an area in the analog form that has a patternwhich will serve the purpose and an original digital representation ofthe pattern that can be compared with the pattern as recorded from theanalog form. It will thus be possible in some cases to use a preexistingpattern in an analog form for the technique. More usually, though, theVAP will be included as part of the design of a new analog form. Thereis of course no need to hide the VAP in the analog form, and indeed insome cases, its presence may be advertised to reassure customers thatillegitimate analog forms can be detected. On the other hand, the VAPcan have any shape, and thus can easily be built into other features ofthe analog form. FIG. 11 shows two examples. At 1101 is shown a barcodewhose bars make up the VAP. At 1103 is a logo which contains the VAP.There may of course be more than one VAP in a document and more than oneVAP may share a location. This can be done by giving each pattern aweighted value such that the weights of all of the patterns sum up toone, e.g.:Final_Pattern=a*pattern1+(1−a)*pattern2, where 0<a<1

One application of multiple patterns would be the authentication ofcontracts, where each party adds its own pattern when it signs thecontract or otherwise terminates a stage in the negotiations.

It is also possible to insert several CDPs on a document at differentplaces, typically produced with different keys, to enable multipleparties to verify their own CDP without being able to verify the CDP ofthe other parties (and consequently being able to duplicate them). It iseven possible to generate a CDP using different keys (each key maycontrol different spatial or frequency area of the CDP), to enabledifferent parties to verify the CDP. This way, if one party releases hiskey, this key is not sufficient to make an exact duplication of the CDP(all keys are necessary), and the security is not compromised. This isanalogue to the concept of “Shared Secrets”.

Registration of the VAP

The preferred embodiment employs black box 106 as registration for theVAP. However, many other registration techniques are possible. Forexample, one could use visible patterns such as frames, bar codes, orthe like already displayed on the package to locate the VAP, as well asOCR. One can also use UV marks or any techniques discussed in the parentpatent application U.S. Ser. No. 10/287,206, J. Zhao, et al., Apparatusand methods for improving detection of watermarks in content that hasundergone a lossy transformation, filed Nov. 4, 2002. Also, one couldalso make the Fourier-Mellin transform of the recorded VAP and match itwith the VAP's original digital representation.

For some applications, it is difficult to know if the orientation of thedigital recording of the VAP is correct, or if it should be flippedupside down (180 degrees rotation) before reading. To avoid having toanalyze the VAP one time, and then, if the analysis is not successful,to rotate it in the opposite vertical orientation and analyze it again,it is possible to design a symmetric VAP: the lower part is a mirror ofthe upper part. The VAP can then be analyzed independently of itsvertical orientation.

Properties of the VAP's Pattern

The pattern can be a grayscale pattern or it can be a colored pattern.In the latter case, different color channels can be employed, forexample RGB and YUV. The pattern can also be generated in variousfrequency domains, for example spatial, wavelet, DFT, or DCT domains.

Generating the VAP

The noisiness, i.e., random nature, of the VAP is what makes itdifficult for counterfeiters and forgers to deal with it. Any techniquewhich can produce a random or pseudo-random pattern will do to generatethe VAP. In the preferred embodiment, generation is done by providing avalue to a pseudo-random number generator which generates a sequence ofrandom numbers that is unique for the value. The value thus serves as akey which may be used to generate new copies of the pattern. Differentpseudo-random number generators may be used in different embodiments andthe probabilistic frequency values for the generated random numbers canbe taken from different probability distributions. The key can also beused to determine the locations in the VAP upon which analysis isperformed. As will be explained in the discussion of using the VAP tocarry other information below, the key may include such otherinformation. In some applications, the key used for designing thepattern may not be revealed to other parties. In that case, any usefulway of distributing keys may be used, for example asymmetric keys orpublic-private key pairs.

The pattern may be combined with a logo, either by adding the logo tothe pattern or vice-versa. The logo can be any existing image ordocument, including images serving other purposes (a 2-D bar code, awatermarked image, etc.). It is also possible to apply any process suchas filtering to the pattern or to the logo in such a way that the logowill minimally interfere with comparing the recorded VAP with theoriginal digital representation of the VAP.

Printing the VAP

The quality of the authentication provided by a VAP depends completelyon the fidelity with which the VAP is printed on the document.Authentication errors can be reduced if a “quality control” step isadded at the end of the printing process to guarantee the fidelity ofthe VAP:

-   -   1. each printed VAP will be passed to an automatic verification        process to check if the authentication pattern has the minimum        quality which is required for it to be recognized as an        original.    -   2. If the quality is below the minimum quality, an alert will be        issued and the document/package containing the authentication        pattern will be re-printed.    -   3. Such verification can also serve as a “quality control” for        printing quality or errors introduced by the printer.

The generation of the VAP can be adapted to the printing technology. Forexample, if a laser printer printing only binary dots is used, then abinary dot VAP can be generated to better use the possibilities of theprinter. Also, a VAP might be more adequately generated, and printed, inthe color space of the printer. If a certain printer uses specific inks(eg CMYK), it can be more effective to generate the VAP in that domainthan in the RGB domain. If the VAP is engraved in metal with a laserengraver able to produce only binary dots, then it would make more senseto generate a binary VAP.

Using the VAP to Carry Other Information

Three approaches to using the VAP to carry other information arediscussed in the following: reserving certain areas of the VAP to holdinformation, using the other information to generate the key used tomake the original VAP, and adding a watermark to the VAP. Thedisadvantage of adding a watermark is that it reduces the ability of theVAP to detect non-original analog forms or modifications in the VAP.

Reserving Areas in the VAP to Hold Information

Certain areas (e.g. 8×8 blocks) of the VAP can be reserved to holdinformation. In those areas, the structure/characteristics of the VAP isnot actually used to verify its authenticity, but to store some bits ofinformation. These areas can be selected pseudo-randomly using a key,such that an entity which does not have the key cannot determine whetheran area in VAP is actually used to store information or to determine theauthenticity of the VAP. In an area that is used to hold information, acertain structure/characteristics of the VAP can correspond to a certainbit value (‘0’ or ‘1’) of information. This bit-dependentstructure/characteristics can of course vary as determined by the key.Note that the reserved areas and the information they contain are partof the VAP as generated. They thus do not degrade the ability of the VAPto detect unauthentic documents. One use of the reserved areas it tostore the key used to generate the VAP.

Using the Information to Generate the VAP's Key

This discussion uses the following terminology: The VAP is created anddetected with a key P; we may want to use a different key S to embed amessage in the pattern as described either above with regard to thereserved areas or below with regard to watermarks; a message M isembedded in the VAP using the key S; finally additional information Imight be printed visibly on the document (serial number, barcode, etc.),or UV-coded invisibly, within the pattern or outside of it, or beobtained from an external source.

Fixed Pattern Key

In one embodiment, the VAP creation key is fixed P. This is typicallythe case for standard offset printing technology, where the printingtechnology does not have the ability to change the pattern dynamicallyfor each package/product/document. The key can be kept secret asdescribed above or may be incorporated into other security features. Forexample, it could be printed in UV inks on the document. The fixedpattern key can be used for brand protection or document protectiongenerally.

Variable pattern key In another embodiment, the VAP's key depends on asecret key S and some other information L This other information I maybe displayed on the document (within the pattern or outside of it) orobtained from an external source. The information from the document canbe for example a serial number, a text, a barcode etc. Information froman external source may for example be a value that is associated withthe VAP and known to the person who is checking whether the documentcontaining the VAP is authentic. The pattern key may be any arbitraryfunction P=ƒ(S,I) of the parameters that are the secret key and theinformation I. A simple function would be to concatenate or sum the twoparameters, but many other functions are possible, such as a hash valueof a combination of the two parameters, etc. At detection time, theprinted information I is extracted with an appropriate technology—barcode reader, OCR, etc—. Then the pattern key is generated as P=ƒ(S,I),and the pattern is analyzed. Typical uses include brand protection withdigital printing.

Watermarks in the VAP

It is possible to embed a visible or invisible watermark in the VAPusing any watermarking technique. The watermark may serve multiplepurposes. It may contain any information, including only a single bit,as described above, or aid registration of the pattern. The watermarkcan either be detected with the key used to generate the VAP or withanother key such that its reading is restricted to another user or groupof users. A third possibility, explained below, is to use the messagecarried by the watermark to derive the key used to generate the VAP.

When a digital watermark is embedded into to a VAP, the VAP will beslightly modified. As a result, when the same VAP is used forauthenticity verification, its reliability for that purpose may bereduced. As an alterative, the digital watermark can be embedded intoareas in the VAP that are reserved to store information as explainedabove.

Watermarks and Keys

In another embodiment, the pattern creation key P is derived from thesecret key S and the message M embedded as a digital watermark in thecopy detection pattern. In this case, M takes the place of theinformation I used to create the variable pattern key discussed above.At creation time, the pattern key P can be any function of the secretkey S and the message M, g(M,S). The pattern is generated in the usualway, then a watermark is inserted into the pattern, where the watermarkencodes the message M using the secret key S as a parameter. Atdetection time, first the watermark message M must be read from thepattern with the secret key S. Once M is known, the pattern key P=g(M,S)is derived, and the pattern is analyzed.

In this application framework, no auxiliary technology would be neededto extract more information printed on the package. It is howeverpossible to also use the information I printed on the package in severalways within the principle described here. For example, the secret key Scan be used in combination with the information I to produce awatermarking key W. i.e. h(S,I)=W, which is used to embed the message inthe pattern. Then the pattern key is generated in the same way asbefore, P=ƒ(M,W)=ƒ(M,h(S,I)). In general, VAPs may be combined withwatermarking technology and other reading technology (OCR or barcodereaders, for example), are to produce different levels of verification.

Comparing VAPs

How recorded VAPs are compared with original digital representations ofVAPs will depend on how the VAP is made and what its purpose is. Somegenerally-applicable variations include evaluating certain areasindependently, either to have more clues on what process has beenapplied to the document or for security features. As described above, aVAP may contain more than one authentication pattern, and the differentpatterns may be analyzed by different groups.

Before VAPs can be meaningfully compared, the comparison program mayhave to be “trained” with VAPs recorded from original documents, asdescribed above for CDPs. The training establishes the thresholds fordetermining whether a VAP recorded from a document whose authenticity isbeing examined is authentic or not. The meaning of the threshold will ofcourse depend on the kind of alteration that the VAP is being used todetect. Retraining is required whenever the manner in which the originaldocuments are printed varies in a manner which affects VAP comparison.Training can be done automatically by printing a number of VAPs on asheet of paper, scanning the sheet, and providing the scan to trainingsoftware.

In another embodiment, instead of comparing the digital recording of atest VAP to a corresponding digital representation to measure itsquality index, it is possible to compare the digital recording to adigital recording of another VAP (typically an original VAP that wasscanned).

Environments in Which VAP Analysis is Performed

What is required to do VAP analysis is a device that can record the VAPfrom the document to make the recorded VAP, a copy of the originaldigital representation of the VAP, and a processor which can compare therecorded VAP with the original digital representation of the VAP. Therecorder and the processor may be local to one another or connected by anetwork. The network may be either a local area network (LAN) or a widearea network (WAN). An example of a local environment is a processor isa PC that has a scanner, a copy of the analysis code, and a copy of theoriginal digital representation of the VAP. The copy of the originaldigital representation of the VAP may either be downloaded or generatedlocally using a key. Results of the analysis are output to the PC'sdisplay device.

In a network environment, scanning, analysis, and the original digitalrepresentation of the VAP may be distributed across the network in anyfashion. A distribution that maintains the security of the originaldigital representation of the VAP and simplifies the equipment needed atthe local level is one in which scanning is done in a device which isconnected to a WAN. When the VAP on the document has been scanned toproduce the recorded VAP, the recorded VAP is sent to a location in theWAN at which both the analysis code and an original digitalrepresentation of the VAP are available. The original digitalrepresentation may be either stored or regenerated on demand. Theanalysis is done at that location and only the result of the analysis isreturned via the WAN to the device used for scanning. In networkenvironments generally, information carried in or sent with the recordedVAP may be used to retrieve information for use in the analysis. Forexample, the document may contain a serial number, and the serial numbermay be sent with the recorded VAP to the location that does theanalysis. If there is an association between VAPs and serial numbers,the serial number could be applied to a database at the location orelsewhere in the network to retrieve the either the key for the originaldigital representation of the VAP that should be compared with therecorded VAP or a copy of the original digital representation of the VAPitself. As described above, the serial number could be specified in abarcode that contained the VAP, as a visible watermark in the VAP, couldbe OCR'd from the document, or even could be input by the person doingthe scanning.

A camera (webcam, camcorder, etc.) can be also used to capture images ofthe VAP. In this case, the VAP detector receives not only one image asinput, but a constant stream of images. The additional informationprovided by several images can potentially be very useful in theanalysis. However, as the time required to analyze one image can besignificantly larger then the time between two successive images, theuse of the stream of images can be optimized. For example, images thatappear to have the properties for a correct reading (good sharpness, VAPwholly contained in the picture), can be selected from the stream andused for analysis.

Combination of VAPs with Other Security Technologies

A VAP can be combined with other technologies targeted at making analogforms more secure. For example, the VAP can be used with informationhiding techniques such as digital watermarking, with machine-readableinformation such as 1-D or 2-D bar codes, with holograms, or with anyother technology that can be applied to an analog form. The relationshipbetween technologies can be multifarious: as an example a 2-D bar codecan contain independent information, or the secret key needed for thepattern analysis, or inversely, the VAP can hold the key required fordecoding the 2-D bar code or the 2-D bar code can contain the VAP.

CONCLUSION

The foregoing Detailed Description has disclosed to those skilled in therelevant technologies the inventors' techniques for determining whetheran analog form of an object is an original analog form or a non-originalanalog form, their techniques for using VAPs to perform authenticitychecks on analog forms, and their techniques for using VAPs to hidemessages in analog forms and has further disclosed to those skilled inthe relevant technologies the best mode presently known to the inventorsfor practicing the techniques. It will be immediately apparent to thoseskilled in the relevant technologies that many embodiments ofApplicants' techniques other than those disclosed herein are possible.For example, the size, shape, and pattern of a VAP will be determined bythe nature of the analog form the VAP is being used with and by theVAP's purpose. How a VAP carries additional information and what thatinformation is will also be determined by the nature of the analog formand by the VAP's purpose. In general, VAPs may be used in any situationin which changes made after the original analog form is made are to bedetected. While the application discloses VAPs printed on documents,analogues to these printed VAPs may be placed on analog forms in othermedia.

For all of the foregoing reasons, the Detailed Description is to beregarded as being in all respects exemplary and not restrictive, and thebreadth of the invention disclosed here in is to be determined not fromthe Detailed Description, but rather from the claims as interpreted withthe full breadth permitted by the patent laws.

1. A method for use with an authenticator, the method comprising:creating an original digital pattern containing an authenticationpattern; producing at least one original analog form from the originaldigital pattern causing a first loss of information in the at least oneoriginal analog form; making a digital recording from at least a portionof the at least one original analog form or from at least a portion ofanother analog form derived from the at least one original analog formcausing a second loss of information in the recorded portion; comparingusing a computer the digital recording with a corresponding portion ofthe original digital pattern using the sum of the first and second lossof information to determine a degree of dissimilarity between thedigital recording made from the portion and the corresponding portion ofthe original digital pattern; and using the degree of dissimilarity todetermine at said authenticator whether the recording was made from theat least one original analog form or from the other analog form.
 2. Themethod set forth in claim 1 wherein the method is practiced in a node ina network and the method further comprises: receiving the digitalrecording from another node in the network, the recording being madefrom the at least one original analog form or from the other analogform.
 3. The method set forth in claim 2 wherein the method is practicedin a node in a network and the method further comprises: returning anindication to another node whether the digital recording has beendetermined being made from the at least one original analog form or fromthe other analog form.
 4. The method set forth in claim 3 wherein themethod is practiced in a processor to which a digital recording deviceand an output device are attached; and the method further comprises:making the digital recording from input received from the digitalrecording device; and providing an indication whether the digitalrecording has been determined being made from the at least one originalanalog form to the output device.
 5. The method set forth in claim 1wherein: in the step of determining a degree of dissimilarity, what isdetermined is dissimilarity of features in the digital recording of theportion and the original digital pattern of the portion, thedissimilarity being caused by operations comprising digital recordingand printing involved in making a non-original analog form.
 6. Themethod set forth in claim 1 wherein: the original digital pattern of theportion has a visible noisy pattern which is perceptible in the analogform by humans.
 7. The method set forth in claim 6 wherein the noisypattern is made using a key and the method further comprises: using thekey to generate the original digital pattern.
 8. The method set forth inclaim 6 wherein: the noisy pattern has a function in the analog form inaddition to permitting determination of whether the analog form is theat least one original analog form.
 9. The method set forth in claim 8wherein: a message may be derived from the noisy pattern.
 10. The methodset forth in claim 9 wherein the method further comprises: using themessage to derive a key; and using the key to generate the correspondingportion of the original digital pattern.
 11. The method set forth inclaim 9 wherein: the message is in portions of the noisy patternreserved there for.
 12. The method set forth in claim 8 wherein: atleast part of the noisy pattern is in a background image.
 13. The methodset forth in claim 8 wherein: at least part of the noisy pattern is in abarcode.
 14. The method set forth in claim 1, wherein the originalanalog form contains a visible noisy authentication pattern.
 15. Themethod set forth in claim 1, wherein the original digital pattern is animage and the step of producing at least one original analog formcomprises printing the image on an object.
 16. The method set forth inclaim 15, wherein the original digital pattern is a grayscale image or acolor image.
 17. A method for use with an authenticator, the methodcomprising: creating an original digital pattern including an originaldigital authentication pattern; making an original analog form thereofincluding an original analog authentication pattern causing a firstloss; recording the original analog form to obtain a recorded originalanalog authentication pattern causing a second loss; comparing using acomputer the recorded authentication pattern and the original digitalauthentication pattern of the original digital pattern using the sum ofthe first and second loss to determine a dissimilarity between theoriginal digital authentication pattern and the recorded authenticationpattern; and using the result of the comparison to perform theauthenticity check at said authenticator, wherein the dissimilaritycomprises an indication that the recorded authentication pattern wasmade from the original analog form.
 18. The method set forth in claim 17wherein: the result of the comparison indicates a portion of therecorded authentication pattern that has been destroyed in the analogform.
 19. The method set forth in claim 17 wherein: the result of thecomparison indicates a portion of the recorded authentication patternthat is from the original analog form.
 20. The method set forth in claim17 wherein the method includes: generating the original digital patternof the authentication pattern.
 21. The method set forth in claim 17wherein the authentication pattern further contains a message.
 22. Themethod set forth in claim 17 wherein: the result of the comparisonindicates a portion of the authentication pattern that has beenoverwritten by written text.
 23. The method set forth in claim 17,wherein the original analog form contains an original visible noisyauthentication pattern.
 24. The method set forth in claim 23 furthercomprising: adding a registration mark associated with said visibleauthentication pattern; and locating said visible authentication patternin said recording using said registration mark.
 25. A method todetermine whether an analog form is original, the method comprising:creating an original digital representation of a document containing anoriginal authentication pattern; printing the document to produce anoriginal analog form causing a first loss of information; making adigital recording from at least a portion of an analog form, the analogform being the original analog form or a non-original analog formderived from the original analog form causing a second loss ofinformation; comparing using a computer the digital recording with theoriginal digital authentication pattern using the sum of the first andsecond loss of information to determine a degree of dissimilaritybetween the digital recording and the original authentication pattern;and using the degree of dissimilarity to determine whether the analogform is original or non-original.
 26. A method for authenticating anobject, the method comprising: creating an original digital image;printing at least one original analog image from the original digitalimage on an object causing a first loss of information; making a digitalrecording from at least a portion of the at least one original analogimage or from at least a portion of a non-original analog image derivedfrom the at least one original analog image causing a second loss ofinformation; comparing using a computer the digital recording with acorresponding portion of the original digital image using the sum of thefirst and second loss of information to determine a degree ofdissimilarity between the digital recording made from the portion andthe corresponding portion of the original digital image; and using thedegree of dissimilarity to determine whether the recording was made fromthe at least one original analog image or from the non-original analogimage.
 27. A method for use with an authenticator, the methodcomprising: creating an original digital pattern containing anauthentication pattern; producing at least one original analog form fromthe original digital pattern causing a first loss of information; makinga digital recording from at least a portion of the at least one originalanalog form or from at least a portion of another analog form derivedfrom the at least one original analog form causing a second loss ofinformation; comparing using a computer the digital recording with acorresponding portion of the original digital pattern using the sum ofthe first and second loss of information to determine a degree ofdissimilarity between the digital recording made from the portion andthe corresponding portion of the original digital pattern; and using thedegree of dissimilarity to determine at said authenticator whether therecording was made from the at least one original analog form or fromthe other analog form; wherein comparing the digital recording with acorresponding portion of the original digital pattern comprisessynchronizing the digital recording with the corresponding portion ofthe original digital pattern using synchronization points in the digitalrecording.